Some electrical equipment, for example, underground mining equipment, is electrically powered using long, heavy electrical cables. As one example, a mining shuttle car used for transporting the mined substance out of the mine stores the electrical cable on a reel. The cable extends from the reel to a cable anchor mounted on the mine wall, and then to a power supply. As the shuttle car or other equipment approaches the cable anchor, the cable is wound onto the reel. As the equipment passes the cable anchor, and begins to travel away from the cable anchor, the movement of the equipment away from the cable anchor applies a tensile force to the cable, actuating a change in the direction of rotation of the reel, unwinding the cable.
Electrical cables are designed to withstand only limited amounts of tensile stress. For example, a typical electrical mining cable is rated to withstand about 200 lb. to about 600 lb. of force. As a shuttle car or other electrical vehicle passes the cable anchor, the tensile stress exerted on the cable may exceed the rating of the cable, causing premature cable wear.
Some presently available cable anchors include elastomeric bands that are intended to stretch in response to a tensile force, thereby reducing the tensile stress on the electrical cable. Other cable anchors utilize a single spring for the same purpose. These devices do not provide for stress reduction over the entire range of force to which the cable anchor may be exposed, and are themselves subject to premature wear. Additionally, many presently available cable anchors channel the cable around a reel that is too small in diameter, thereby bending the cable to a greater degree than the cable is designed to withstand.
Dual spring arrangements are known in the field of semiautomatic firearms, as explained in U.S. Pat. No. 4,204,113 to Seecamp. The spring arrangement disclosed by this patent is directed towards providing sufficient forward bias to the slide of a compact semiautomatic handgun having insufficient space for a single spring relative to the slide's distance of travel. Although the possibility of different spring rates is suggested by this patent, the patent is entirely directed towards maximizing the ability of the spring assembly to fit within the minimized available space when the spring assembly is compressed while also providing sufficient force when the spring assembly is expanded into a greater available space. Nothing within this patent discloses a means of providing tensile stress relief over a very wide range of applied tensile forces.
U.S. Pat. No. 4,040,332 to Border et al. discloses a variable spring rate equilibrator for an artillery piece. The equilibrator utilizes a pair of springs separated by a piston. A stop member prevents movement of the piston beyond a predetermined point as the artillery piece is elevated, so that only one spring is acting on the artillery piece. As the artillery piece is lowered, the piston moves away from the stop member, so that both springs can act on the artillery piece. This patent is therefore directed towards applying or removing the force of one of the two springs based on the position of the center of mass of the artillery piece, and not towards reducing a tensile stress over a wide range of applied tensile forces.
Accordingly, there is a need for a cable anchor that provides for tensile stress reduction over a greater range of tensile forces, that reduces the wear to which the cable is subjected, and which is itself durable.